not enough repo for commenting yet, so writing here, because it's a common thing with jittering near the end-position (not sure if that is your problem because it's not stated in your question)
If your actual camera is approaching the end position and you're applying your next translation/rotation it may overshot by a fraction and in the next frame it does ...
The second camera is the only one rendering to the screen. The "master camera" is rendering to a texture, which means that it will not render to the screen. If you want to allow the camera to render to the screen, you need to set the render texture target to null.
Transmitting the full image contents of the player's screen across the network to the server every frame is likely to be rather performance-intensive, not to mention bandwidth-intensive.
A much more scalable approach is to instead transmit the position and orientation of the player's camera. Then the server can spawn its own camera for each player in its ...
I figured out what it is (with the help of an amazing dev friend of mine)
This line, right here, was causing the issue:
transform.localRotation = Quaternion.AngleAxis(-mouseLook.y, Vector3.right);
character.transform.localRotation = Quaternion.AngleAxis(mouseLook.x, character.transform.up);
Now I've deleted it, it stops the rotation issue. ...